High-voltage PN diodes are generally used for high-voltage applications. Such high-voltage PN diodes advantageously have a low leakage current and a high robustness. Disadvantages of such high-voltage PN diodes are that they have a high forward voltage and a high switching power loss.
In a high-voltage PN diode of this type, the voltage is primarily taken over by the slightly doped region which is provided in such diodes. In the case of operation in the forward direction, electrons and holes are injected into the slightly doped region. At a high current density, high injection prevails in the slightly doped region and the electron and hole densities are greater than the doping concentration of the slightly doped region. This increases the conductivity of the slightly doped region. This advantageously results in a reduction of the forward voltage. However, the current of a high-voltage PN diode at room temperature does not begin to flow until forward voltage UF=0.7 V, for example. Under normal operating conditions, for example, at a current density which is greater than 100 A/cm2, forward voltage UF rises to values greater than 1 V. This is associated with a correspondingly high, undesirable power loss. Since a high-voltage PN diode requires a thick slightly doped region, the voltage drop in the forward direction is relatively great across the slightly doped region despite the conductivity modulation.
The charge carriers (electrons and holes), which are injected into the slightly doped region and stored there during the operation in the forward direction, must be reduced at the time of a shut-off, for example, at the time of an abrupt current commutation, before the high-voltage PN diode is actually capable of taking over the reverse voltage again. For that reason, the current first flows in the reverse direction at the time of an abrupt current commutation until the stored charge carriers are reduced or removed. This process, i.e., the level and the duration of the drain current for reducing the stored charge carriers, is primarily determined by the volume of the charge carriers stored in the slightly doped region. A higher and longer lasting drain current means a higher turn-off power loss.
If high-voltage Schottky diodes are used instead of high-voltage PN diodes in high-voltage applications, the turn-off power loss is advantageously reduced. A high-voltage Schottky diode is a majority charge carrier component, in which no high injection occurs even at high current density during operation in the forward direction, i.e., no injection of electrons and holes into the slightly doped region. However, since no injection with conductivity modulation occurs in the case of a high-voltage Schottky diode, a high voltage drops at the slightly doped region during operation with high currents. This has previously limited the use of high-blocking Schottky diodes to very low currents. Additional disadvantages of high-voltage Schottky diodes are the high leakage currents, in particular at high temperature, as well as their severe voltage dependence due to the barrier-lowering effect.
In German Application No. DE 10 2010 043 088.9, a Schottky diode is described which has the advantage of a low turn-off power loss and, at least partially, overcomes the disadvantages of a high forward voltage and high leakage currents occurring in customary high-voltage Schottky diodes. Such a Schottky diode, which is also referred to as a super trench Schottky barrier diode (STSBD), is shown in FIG. 1. It has an n+-substrate 10, an n-epilayer 20, trenches 30 etched into n-epilayer 20, each of which has a width Wt and a distance D_epi to the n+-substrate, mesa regions 40 having a width Wm between adjacent trenches 30, a metal layer 50 acting as a Schottky contact and used as an anode electrode on front face V of the Schottky diode implemented as a chip, a metal layer 60 acting as a cathode electrode on back face R of the chip, and additional Schottky contacts 70 on the walls of trenches 30. Each of these additional Schottky contacts 70 has a width D_sk in the direction to n*-substrate 10 and a distance D_gap to each next additional Schottky contact 70. Metal layer 50 covers each of the trench walls to a depth D_anode and has a distance D_gap to each adjacent first additional Schottky contact 70. Schottky contacts 70 float on the trench walls. Each additional Schottky contact 70 closest to n*-substrate 10 covers one trench bottom each. Any number of floating contacts may be selected. Preferably, significantly more contacts are selected than are represented in FIG. 1.
In an STSBD of this type, the space charge region in n-epilayer 20 expands in the reverse direction with increasing voltage toward the trench bottom. If the space charge region at a voltage V1 reaches the first floating Schottky contact, this voltage V1 is taken over by the first floating Schottky contact. The space charge region expands with continuously increasing voltage toward the trench bottom. The voltage at the first floating Schottky contact remains unchanged.
In a similar way, the space charge region at a higher voltage Vn reaches the nth floating Schottky contact. In this connection, the nth floating Schottky contact takes over voltage Vn. In turn, with a continuing increase in voltage, the voltage on the nth floating Schottky contact remains unchanged.
If width D_sk and distance D_gap in the STSBD structure are selected to be equal for all additional Schottky contacts 70, a periodically homogeneous field distribution is present in mesa region 40. The field distribution in the mesa region is continuously repeated according to a distance D_sk+D_gap until the trench bottom is finally reached. The voltage distribution in mesa region 40 is then largely linear. Compared to conventional high-voltage PN diodes or high-voltage Schottky diodes, in the case of the STSBD, a significantly higher voltage may be accommodated at a predefined thickness of the slightly doped region.
This results in a very advantageous compromise between breakdown voltage, forward voltage and turn-off power loss.